=Mercuric Ammonium Chloride, Infusible White Precipitate=, HgClNH₂.—This compound, which is required in the last-mentioned process for the manufacture of vermilion, is most cheaply made in the following way: to a solution of 1 part of common salt in 32 parts of water, 2 parts of dry mercuric sulphate are added in small quantities, whilst thoroughly stirring. It is absolutely necessary to work in this way, because mercuric sulphate is decomposed by water into free sulphuric acid and a basic salt, which is much more slowly converted into white precipitate than the neutral salt. The liquid now contains mercuric chloride; on the addition of ammonia to alkaline reaction it gives a heavy white precipitate. The liquid is poured off and the precipitate washed with water containing a little ammonia, until the washings give only a slight turbidity with barium chloride. This process may also be commenced with mercuric chloride, but the above method is cheaper. White precipitate should volatilise without fusing when heated on platinum foil and should keep its white colour when treated with ammonia. =Electrolytic Process.=—In a wooden vessel 1 metre in diameter and 2 metres deep are placed, at the sides, dishes 15 centimetres wide, containing a layer of mercury 1 centimetre deep. These dishes are connected with the positive pole of a dynamo. The negative pole is connected to an iron plate, electrolytically coated by copper, placed at the bottom of the vessel, which is filled with a solution containing 8 per cent. of ammonium nitrate and 8 per cent. of sodium nitrate. A regular current of sulphur dioxide is introduced through a perforated coil. The excess of gas escapes by a tube in the cover. When the current is passed a precipitate of red mercuric sulphide is at once formed. Attempts have been made to dispense with the current of sulphur dioxide. The bath then contains 100 litres of water and 8 kilogrammes each of ammonium nitrate, sodium nitrate, sodium sulphide and sulphur. Under these conditions it is only necessary to add sulphur and mercury in order to obtain, at the end of the operation, vermilion which is in no way inferior to that made by the first process. Vermilion is frequently grossly adulterated by cheaper pigments. Substances are often found under the name of vermilion which contain no trace of mercury, but consist of bright orange lead mixed with a few per cent. of ferric oxide, and having a deceptive similarity in colour to the best vermilion.